A cyclotron is a type of circular particle accelerator in which negatively or positively charged particles are accelerated outwards from the centre of the cyclotron along a spiral path up to energies of several MeV. In isochronous cyclotrons, the particle beam runs each successive cycle or cycle fraction of the spiral path in the same time. Unless otherwise indicated, the term “cyclotron” is used in the following to refer to isochronous cyclotrons. Cyclotrons are used in various fields, for example in nuclear physics, in medical treatment such as proton therapy, or in radio pharmacology. In particular, cyclotrons can be used for producing short-lived positron-emitting isotopes suitable for PET (positron emitting tomography) and SPECT imaging (single photon emission computed tomography).
A cyclotron generally comprises several elements including an injection system, a radiofrequency (RF) accelerating system for accelerating the charged particles, a magnetic system for guiding the accelerated particles along a precise path, an extraction system for collecting the thus accelerated particles, and a vacuum system for creating and maintaining a vacuum in the cyclotron.
A particle beam is introduced into a gap at or near the center of the cyclotron by the injection system with a relatively low initial velocity. This particle beam is sequentially and repetitively accelerated by the RF accelerating system and guided outwards along a spiral path comprised within the gap by the magnetic field generated by the magnetic system. When the particle beam reaches its target energy, it is extracted from the cyclotron by the extraction system provided at a point of extraction. This extraction system can comprise, for example, a stripper consisting of a thin sheet of graphite. For example, ions passing through the stripper lose two electrons and become positive. Consequently, the curvature of their path in the magnetic field changes its sign, and the particle beam is thus led out of the cyclotron towards a target. Other extracting systems exist which are well known to the persons skilled in the art.
The magnetic system generates a magnetic field that guides and focuses the beam of charged particles along the spiral path until it is accelerated to its target energy (cf. FIGS. 4&5). In the following, the terms “particles”, “charged particles”, and “ions” are used indifferently as synonyms. The magnetic field is generated in the gap defined between two magnet poles by two solenoid coils wound around these poles. Magnet poles of cyclotrons are often divided into alternating hill sectors and valley sectors distributed around a central axis. The gap between two magnet poles is smaller at the hill sectors and larger at the valley sectors. A strong magnetic field is thus created in the gap within the hill sectors and a weaker magnetic field is created in the gap within the valley sectors. Such azimuthal magnetic field variations provide radial and vertical focusing of the particle beam. For this reason, such cyclotrons are sometimes referred to as sector-focusing cyclotrons. In some embodiments, a hill sector has a geometry of a circular sector similar to a slice of cake with a first and second lateral surfaces extending substantially radially towards the central axis, a generally curved peripheral surface, a central surface adjacent to the central axis, and an upper surface defining one side of the gap. The upper surface is delimited by a first and second lateral edges, a peripheral edge, and a central edge (cf. FIGS. 1(b) and 3).
In order to maintain a vacuum in the gap and to control and contain the magnetic field in the space surrounding the gap and pair of magnet poles, a cyclotron generally also comprises a yoke. A yoke is formed by a first and second base plates normal to the central axis, Z, which are separated from one another by a flux return yoke. The first and second base plates and flux return yoke define together a chamber, with the flux return yoke forming the outer walls of the cyclotron and controlling the magnetic field outside of the coils by containing it within the cyclotron. The first and second magnet poles are contained within the chamber. The first and second base plates are provided with openings for fluid communication of the chamber with vacuum pumps.
The flux return yoke is generally formed of two parts which are joined at the level of a median plane normal to the central axis, Z, so that the cyclotron can be opened by moving the first base plate and flux return yoke first part, together with the first magnet pole away from the second base plate, flux return yoke second part and second magnet pole. The flux return yoke must have a minimal thickness, Tv, in order to close and to contain within the cyclotron the magnetic field generated by the magnet poles outside the gap.
A cyclotron is typically a massive and voluminous piece of equipment weighing several tens of tons. This generally has an impact on the production cost as well as on the cost of transportation and handling of a cyclotron. Standard intermodal containers have a width of about 2.4 m and a similar height, with larger containers such as 40′- and 45′-high-cube containers, reaching a height of about 2.7 m. In order to fit in a standard intermodal container, a cyclotron must fit in a crate of less than 2.4 m (or 2.7 m). The dimensions of a low energy cyclotron, such as one suitable to accelerate 18 MeV protons, usually exceeds the size of standard intermodal containers, with a yoke of diameter of about 2 m and a hydraulic system positioned outside of the yoke. The high volume of cyclotrons requiring the use of non-standard containers together with the high weight of cyclotrons may have a negative impact on the cost and handling of cyclotrons.
There therefore remains a need in the art to provide an isochronous sector-focused cyclotron of both lower weight and lower dimensions, to reduce the costs of production and transportation and to enhance the ease of handling of such cyclotrons. Embodiments of the present disclosure may offer a solution for reducing considerably the volume and weight of cyclotrons.